1. Field of the Invention
The present invention relates to electrochemical energy converters such as fuel cells or electrolysis cells. More particularly, it relates to Proton Exchange Membrane Fuel Cells (PEMFCs). The present fuel cell invention can be categorized further as relating to PEMFCs constructed, in part, of separators, each comprised of a respective series of conductive compression gaskets possessing inter-related fluid distribution channel and manifold features.
2. Description of the Prior Art
The class of electrochemical fuel cells comprising Proton Exchange Membranes (PEMs) convert the chemical energy of a hydrogen fuel and an oxidant into electrical energy, heat and water. Generically, PEM fuel cells employ Membrane Electrode Assemblies (MEAs) comprising a solid polymer electrolyte or ionomeric membrane interposed between an anode and a cathode, each electrode comprising electrocatalyst, typically Platinum (Pt). The electrocatalyst depositions define the fuel cell “active area” as the location in which the electrochemical reactions occur between the reactants and the Pt electrocatalyst. In turn, the MEA is interposed between an anode separator and a cathode separator; each separator comprising electrically conductive material and the physical means for fluid distribution of the reactants throughout the fuel cell.
As the need for sustainable energy supplies increases, PEM fuel cells are an appealing energy source because they convert universally abundant hydrogen and oxygen into energy in a practically noiseless electrochemical process that produces only electricity, heat and water. Yet, heretofore, technical and economical difficulties have abated the realization of PEM fuel cell technology as a viable commercial means for producing energy. Both the technical and economical difficulties originate from material issues concerning the materials employed within PEM fuel cell embodiments.
One of the most considerable material issues obstructing the commercial development and deployment of PEM fuel cells originates with the materials comprising the separators. Prior art PEM fuel cells typically comprise either metal or graphite separators. Ordinary metallic separators, fabricated from aluminum for example, will corrode and produce minute metal particulate that will react with the electrocatalyst and terminally diminish or arrest the electrochemical reactions. In order to prevent what is known as “catalyst poisoning,” the PEM fuel cell industry plates the metallic separators with gold or exotic alloys, or employs metallic separators comprising expensive alternatives such as titanium. While the use of gold-plated, alloy-plated or exotic separators can prevent electrocatalyst poisoning, it is not cost effective for the large-scale commercial market.
Graphite separators are more cost effective than metallic separators because they are made from a less expensive material, and they are more chemically compatible with the electrochemical reaction but they present several technical complications concerning material integrity. Graphite separators typically are machined from monolithic slabs of graphite or sintered from loose graphite material. Machining graphite separators often requires tortuous tooling paths and complicated construction geometries that make the separators susceptible to technical problems during operation, such as crossover and overboard leakage, incongruous components and compromised structural integrity, etc. The sintering process possesses inherent variability that can translate into imperfections in sintered graphite separators. The loose graphite material is formed into a separator under thermal compression; this operation is susceptible to non-uniform compression and thermal gradation, which can translate into material imperfections and structural integrity problems, such as internal voids, surface defects and cracking. In addition, both machining and sintering are relatively expensive manufacturing methods and require long part-fabrication times. Moreover, graphite separators inherently exhibit lower thermal and electrical conductivity than metallic separators, thereby limiting the potential electrical generation of the fuel cell.
From a technical perspective, a significant shortcoming in the development of PEM fuel cells for commercial production is the inability to prevent reactant leakage between fuel cell components. Typically, PEM fuel cells are embodied as stratified apparatus comprising multiple planar components: the anode and cathode separators, the MEA and an elective cooling separator. The interfaces between the fuel cell components are susceptible to reactant leakage because the complementary surfaces are not completely mutually conformable. Crossover leaks (from one internal fluid stream to another) and overboard leaks (from interior fluid streams to the exterior environment) inhibit the performance, efficiency, power density, stoichiometry, etc. of the fuel cell system. Therefore, sealing materials, such as resilient gaskets are often incorporated into conventional fuel cell constructions. Gaskets are incorporated around the perimeter of the fuel cell in order to minimize or arrest over-board reactant leakage into the external environment. In addition, gaskets are employed to circumscribe the manifold and channel features and the active area in order to prevent the internal commixture of the reactants. However, the employment of gaskets within the fuel cell is a qualified solution because it also complicates several issues, such as the fuel cell design, construction, material compatibility and product life expectancy.
Moreover, fuel cell embodiments that employ graphite separators typically incorporate non-active cooling cells intermittently within the fuel cell stack rather than within individual fuel cells. This type of PEMFC construction is susceptible to thermal gradients, which are detrimental to the performance of the fuel cell stack and exacerbates water managements issues of dehydration and flooding.
These deliberations elucidate several specific difficulties inherent in PEM fuel cells that are specifically relevant to the present invention. Accordingly, it is evident that it is necessary to improve upon the PEM fuel cell design, the PEM fuel cell construction and the materials utilized in the embodiment in order to surmount the limiting difficulties of PEM fuel cell prior art.